Magnesium (Mg) has a specific gravity of 1.74 g/cm3, which is ⅔ of the specific gravity of aluminum and ⅕ of the specific gravity of steel. Magnesium is the lightest metal among structural metals currently used, and is an environmentally friendly material having an unlimited resource because magnesium has high specific strength and is easily recycled. Since magnesium is an eighth most abundant element on earth, accounting for about 2.7% of the earth's crust, and in particular, magnesium constitutes about 0.13% of seawater, magnesium may be considered as an infinitely available resource.
Use of magnesium has been gradually increased according to demands for lightweight transportation vehicles in consideration of the global environment and fuel economy efficiency, and thus the application of magnesium to 3Cs products such as a mobile phone and a notebook, continues to increase to meet the demands for lightness, thinness, shortness, and smallness, and electromagnetic wave shielding property. Accordingly, study on material processing of magnesium alloys has been very actively conducted in various fields such as military/defense, transportation, and 3Cs.
Since magnesium parts or facilities are used in a service environment where a repetitive load or deformation is applied, a magnesium alloy processing material should have excellent fatigue properties in order to be applicable to various fields with high reliability. However, solutions for the above are incomplete so far. In particular, when compared with aluminum (Al), which is a major competitive material as a lightweight material, magnesium has a problem in that its applicability is limited due to fatigue properties inferior to those of aluminum because of low fracture toughness and, especially, poor fatigue properties at a low cycle fatigue region.
Meanwhile, the related arts related to magnesium alloys are described below. Korean Patent Application Laid-open Publication No. 2007-0114621 discloses a technique of controlling a temperature of a rolling roll and a surface temperature of an alloy plate according to the content of aluminum present in a magnesium alloy in order to obtain a magnesium alloy plate having excellent plastic workability, e.g., press workability.
However, this patent is disadvantageous in that material applicability may be limited because Al content is limited to mass percentage, and is also problematic in that the surface temperature of the magnesium alloy plate should be increased.
Also, Korean Patent Application Laid-open Publication No. 2008-0104721 discloses a magnesium alloy having higher strength and toughness than typical magnesium alloy, in which the homogeneity of a microstructure is improved by preventing the segregation of magnesium alloy through the addition of manganese, zirconium, zinc, and copper. This patent still has disadvantages in that cost is inevitably increased due to the addition of expensive alloying elements and a processability problem after preparation of the alloy is not resolved yet.
In addition to the above-described patents, patents regarding magnesium alloys relate to a magnesium alloy composition system and a method of processing magnesium alloy, and most of applications for those patents have been led by Japan. However, it is very difficult to find out a technique for improving fatigue properties of a magnesium alloy.